The search for new oil or gas resources has now reached a stage where it is moving away from relatively easily accessible continental shelf waters and towards deeper waters. This gives rise to technical challenges including the problem of gas hydrate deposition in pipelines and in production facilities. Gas hydrate is an ice-like compound consisting of light hydrocarbon molecules encapsulated in an otherwise unstable water crystalline structure. These gas hydrates form at high pressure and low temperature wherever a suitable gas and free water are present. Gas hydrate crystals can deposit on pipelines walls and in a production facility, and in worst case scenarios can lead to complete blockage of pipelines or the vessels and flow lines of the production facility. Although gas hydrate formation is a major problem for gas production, the formation of gas hydrates is also a problem for gas condensate and crude oil production.
There is a growing understanding in the oil and gas industry that gas hydrate particles in a flow situation are not necessarily a problem per se. If the particles do not deposit on pipeline walls or equipment and do not have a significant impact on fluid flow behavior (i.e. their concentration is not too high), the particles simply flow with the rest of the fluids. Thus, U.S. Pat. No. 6,774,276 describes a method for transporting a flow of fluid hydrocarbons containing water through a treatment and transportation system including a pipeline, the method comprising:    introducing the flow of fluid hydrocarbons into a reactor wherein the flow of fluid hydrocarbons contains water;    introducing a cold fluid flow of hydrocarbons, containing particles of gas hydrates acting as a hydrophilic agent, into the reactor where it is mixed with the flow of fluid hydrocarbons containing water;    cooling an effluent flow of hydrocarbons from the reactor in a heat exchanger to ensure that free water present therein attains the form of gas hydrates;    treating the cooled effluent flow in a separator to separate the flow into a first flow and a second flow, wherein the first flow has a content of gas hydrate;    recycling the first flow to the reactor to provide the particles of gas hydrate; and    conveying the second flow to a pipeline to be transported to a destination.
By seeding the flow with gas hydrate particles, hydrate growth takes place on the seed particles. The gas hydrate particles increase in size but remain entrained in the flow and therefore do not deposit on the walls of the pipeline. The gas hydrate particles will not melt back to free the water and natural gas until temperatures rise or pressures become too low —which in reality will be at the end of the pipeline. According to U.S. Pat. No. 6,774,246, the hydrate powder can be mechanically separated from the bulk liquid phase by a sieve. Another method would be to melt the hydrates in a separator where the residence time is long enough for the emerging water to separate out from the hydrocarbon liquids. Also, depending on the fluid system, the particle density may even deviate enough from the bulk liquid so that the particles may easily be separated off. However, there remains a need for an improved process for the regasification of gas hydrate particles that are entrained in a multiphase produced fluid.
International patent application number WO 97/24550 relates to a terminal plant and a method according to which a hydrocarbon product which may consist of only hydrate or may consist of a suspension of a carrier liquid and gas hydrate suspended therein, is potentially stored for a certain time before being dissociated so that a gas is generated for further transport or use. The hydrocarbon product is stored within one or several storing tanks at a temperature so low and stable that the hydrate is maintained in form of hydrate at the storing pressure which possibly may be very close to the normal atmospheric pressure. This allows the storage tank or tanks to be built without any reinforcing structures and thick walls. Such storing tanks are used in a plant together with at least one dissociation tank which may have a much smaller volume than the storing tank or tanks, and such dissociated tank or tanks are dimensioned to withstand a pressure which corresponds to the degasification pressure for the released gas when the hydrate dissociates, which in practical terms means a pressure of approximately 50 to 60 bars. It is said to be advantageous if the hydrocarbon product is in the form of a suspension comprising relatively small particles of gaseous hydrate suspended in a carrier liquid which preferably consists of hydrocarbon liquid or of a mixture of different hydrocarbon liquids, preferably mainly of a non-hydrate-forming nature. One of the objects of the carrier fluid is to give buoyancy to the gas hydrate particles which substantially reduces or completely prevents the tendency of compaction of the hydrate in the lower parts of the storing tank. In contrast, the present invention does not store the multiphase fluid that contains the gas hydrate particles within a storage tank prior to regasifying the gas hydrate particles. In addition, the process of WO 97/24550 is not capable of handling the large volumes of gaseous hydrocarbon and liquid hydrocarbon that are formed in the process of the present invention.